RF Power Amplifier Performance by Clipping Prevention of Large PAPR Signals

ABSTRACT

Preventing RF signal distortion and signal error producing memory events in a Radio Frequency (RF) power amplifier (RFPA). An element, disposed prior to the Radio Frequency (RF) power amplifier (RFPA) in a signal path of a RF signal input to the RFPA, may enforce a maximum allowable amplitude in a high PAPR instantaneous high peak of the RF signal. An element may also increase or supplement a bias of the Radio Frequency (RF) power amplifier (RFPA) when a high PAPR instantaneous high peak is detected in the RF signal prior to receipt by the RFPA. Additionally, a first element operable detects when an instantaneous output voltage of the Radio Frequency (RF) power amplifier (RFPA) is below a predetermined voltage, and in response, a second element supplies additional current to prevent the output voltage of the RFPA from falling below a predetermined threshold voltage.

CLAIM OF PRIORITY

The present application is a continuation of, and claims priority to,U.S. non-provisional patent application Ser. No. 16/780,147, entitled“Improving RF Power Amplifier Performance by Clipping Prevention ofLarge PAPR Signals,” filed Feb. 3, 2020, the disclosure of which ishereby incorporated by reference for all purposes in its entirety as iffully set forth herein.

U.S. non-provisional patent application Ser. No. 16/780,147 claimspriority to U.S. Provisional Patent Application No. 62/802,446, entitled“Improving RF Power Amplifiers Performance by Clipping Prevention ofLarge PAPR Signals,” filed Feb. 7, 2019, the disclosure of which ishereby incorporated by reference for all purposes in its entirety as iffully set forth herein.

FIELD OF THE INVENTION

Embodiments of the invention generally relate to a Radio Frequency (RF)power amplifier (RFPA).

BACKGROUND

The peak voltage of a pure continuous wave (CW) signal is only √2 higherthan its average root mean square (rms) voltage, exhibiting a peak toaverage power ratio (abbreviated as PAPR and expressed in dB) of 3 dB. Asignal composed of N independent equal amplitude CW's has a maximum peakvalue of N×√2 of the average rms value of one CW, and an rms value of √Ntimes the single CW average value. Thus, the maximum PAPR of that signalis 20×log 10(N×√(2/N))=10×log 10(N)+3 dB.

As can be appreciated from this mathematical expression, PAPR growslogarithmically with the number of CW's. The probability forencountering such a high peak value is extremely small. For example, ifN=30, while the max PAPR could be ˜18 dB, the probability that the peakof all independent CWs will align within ±5 degrees is(10/360)²⁹=7×10⁻⁴⁶.

When a RF signal is composed of multiple QAM carriers, the same equationcan be used by using a single QAM carrier PAPR to replace the 3 dB of aCW PAPR. In order to quantify the “reasonable to occur” PAPR value, onehas to evaluate the rate at which the signal is sampled vs. the expectedwait time for a peak to occur. For example, at a sample rate of 5 Gsps,5×10⁹ samples will be taken every second, making it very likely that aprobability of 10⁻⁷ will occur in every wait period of 1 second, butvery unlikely that a probability of 10⁻¹³ will occur in the same waitperiod.

The signal spectrum of several types of broadband communication schemes(such as without limitation wireless, cellular, Wi-Fi, and CATV) iscomposed of deep modulation carriers (for example, QAM and OFDM). Often,that spectrum is composed of multiple such carriers. As a result, the RFsignal carried by these media can suffer from high PAPR.

High PAPR signals exhibit occasional very high instantaneous amplitudepeaks which are much higher than the signal average value. Such behavioris commonly found in wideband communication signals such as those usedin wireless, cellular, Wi-Fi, and CATV systems. The following example,evaluating the nature of a typical CATV signal, is representative ofsignals used by other communication systems. A reasonably occurring peakof a of a typical CATV signal (e.g., about once every second) can be ˜17dB higher than the signal average value. While the probability of suchvery high peak levels occurring in any single sample is very low, theprobability of lower peak values is much higher. For example, for thesame signal, the probability may go up to 10⁻⁴ for PAPR of about 12 dB.

It is customary to plot PAPR probability of a certain signal in a graphwhere the X axis represents PAPR_(Threshold), and the Y axis representsthe probability of any PAPR higher than that PAPR_(Threshold). FIG. 1illustrates such a PAPR plot for a typical CATV signal composed of 128×6MHz channels of 256-QAM modulation in accordance with the prior art. Asshown in FIG. 1, the probability of any PAPR higher than thatPAPR_(Threshold) is inversely proportional to the value ofPAPR_(Threshold).

The design of a communication device should account for the expectedPAPR values of the signals it handles and their likelihood of occurrenceby allowing reasonably high instantaneous peaks to be processed with noor minimal distortion by the various components in the signal path. Forexample, a back off of 16˜17 dB is typically used in a DAC (digital toanalog converter) that processes a broadband CATV signal. In otherwords, the root mean square of the signal is set to be 16˜17 dB belowthe maximum signal amplitude that can be handled by the DAC (thismaximum signal amplitude is also known as the DAC full scale).Amplifiers that are used to provide gain to a broadband CATV signal alsoare deployed with similar considerations in mind, albeit typically at aslightly lower back off amount.

FIG. 2 is a time domain linear plot of a typical CATV(t) signal composedof 128×6 MHz channels of 256-QAM modulation in accordance with the priorart. FIG. 2 depicts about 600,000 digital samples of a 768 MHz broadsignal sampled at about 2.5 Gsps. Note that the signal shown in FIG. 2has both positive and negative peaks.

FIG. 3 is an absolute linear value plot of the same signal(ABS(CATV(t))) shown in FIG. 2 in accordance with the prior art. FIG. 4depicts the same signal shown by FIG. 2 in logarithmic scale (20*log10(ABS(CATV(t)))) in accordance with the prior art. FIG. 4 is scaledwith the RMS value at 0 dB and depicting values which are higher than 8dB above the RMS value. FIG. 5 depicts, in the CATV(t) signal of FIG. 2,100 samples immediately near the highest PAPR in accordance with theprior art. As can be appreciated from viewing FIG. 5, although theprobability of a very high peak is low, it is typical to find severalother high (but slightly lower) peaks not far in value and separated byseveral samples from the very high instantaneous peak. Specifically, inthe signal depicted by FIG. 5, two high peaks, namely peaks 504 and 506,are about 4 dB lower than the highest peak of peak 502, which is at ˜14dB. Peaks 504 and 506 occur in the signal very close to highest peak502.

In Hybrid Fiber Coax (HFC) systems, distributed feedback lasers (DFB)are often used to transport and distribute Cable TV signals across largedistances, beyond the capabilities of electrical coaxial cables. Whenused to convey a wide RF spectrum of modulated CATV signals, theselasers are biased with a certain DC bias current, such that whencombined with the modulating RF signal, the total current to the laserstays in the linear zone for any “reasonable” RF momentary current.Since the RF signal is typically symmetrical in amplitude, the preferredDC current bias is in the middle of the laser linear zone, and thepreferred RF signal current will be adjusted in amplitude such that therange of laser current stays in the laser linear zone. To illustrate,FIG. 6 is a graph of laser bias current vs. optical output power inaccordance the prior art. As shown in FIG. 6, the preferred DC currentbias is in the middle of the laser linear zone.

As is typical in a CATV high PAPR signal, there may be the presence oflarge PAPR instantaneous RF signal peaks that have a low probability ofoccurring. Such large PAPR instantaneous RF signal peaks will cause thetotal laser current to be either lower or higher than the laser linearrange. In response to such occasions, the laser will produce undesirablenon-linear distortions. These distortions will grow in amplitude as thetotal current increases beyond the laser linear range.

It is often desirable to drive higher RF amplitude into the laser, andby that increase the optical modulated index (OMI) of the signal, whichoften results in improved signal reception quality at the receiver. Atthe same time, there exists graceful degradation in the laser linearitywhen the RF signal amplitude grows and drives the laser further outsideits linear range. Since the probability of very high PAPR RFinstantaneous peaks occurring is very low, the average non-linearity mayalso be very low, but instantaneous high non-linearity events may causebit errors in signal demodulation. Thus, a compromise is often employedto balance the desired high OMI with an acceptable level and probabilityof laser non-linearity.

Nevertheless, if at any time the RF signal instantaneous amplitude isover a certain threshold (that threshold being an amplitude which causesthe sum of bias current plus RF signal current to be less than a certainvalue required for the laser to emit light, i.e., “the clipping point”or “clipping threshold”), a catastrophic event occurs. The laser stopslasing for a very short duration of time associated with the duration ofthe instantaneous very large RF signal peak. When the instantaneoussignal goes below that threshold after the very short duration of time(often measured in picoseconds), the laser starts lasing again.

However, the laser lost its light coherency during that shortinterruption in lasing. It takes a relatively long time for the laser toregain its light coherency. This coherency recovery time length is oftenseveral orders of magnitude longer than the length of time of which thesignal instantaneous amplitude was above the “clipping threshold.”During the coherency recovery time, while the laser light may bemodulated by the required signal, the wavelength of the light is outsidethe allowable range. Thus, there exists a very large and long signalerror in the actual received light relative to the expected receivedlight until the laser again regains coherency and its signal is againproperly receivable by the intended receiver. In fact, the loss of lightcoherency creates a “memory effect,” where the high PAPR extreme eventcondition is “remembered” for a time by the laser, and for a time, thelaser output is still affected by the extreme event even after the eventcause is removed from the laser input.

As a result of the laser clipping effect, often the maximum OMI of thelaser is not limited by the “regular” non-linear distortion that resultsfrom operating the laser slightly outside its linear zone, but ratherfrom the probability that a large PAPR instantaneous RF signal peak willcause the laser to lose light coherency. This is so because suchelongated signal error periods are much more likely to cause bit errorsat the receiver and the demodulator than the relatively smallnon-linearity.

A prior art approach uses pre-clipping of the RF signal before it isapplied to the laser. By pre-clipping the signal, the instantaneoussignal amplitude is limited to amplitudes below a certain pre-clippingvalue, and thus is forced to be lower than the value that will cause thelaser to stop lasing. Since laser clipping is a unipolar effect (i.e.,caused only by negative polarity high RF signal peaks), the pre-clippingimplementation only has to limit the RF signal negative peaks and notits positive peaks.

To ensure some operating margin that will ascertain the prevention ofclipping at the laser, the amplitude of the “deliberate signal error”created by the pre-clipping operation will typically be slightly largerthan the extent of the instantaneous signal excursion beyond the laserclipping point if the pre-clipping is not applied. However, thepre-clipping mechanism is designed to be very fast reacting, and thesignal error duration is limited to the time length in which theinstantaneous signal is higher in amplitude than the pre-clippingthreshold. That potentially larger amplitude “deliberate signal error”is orders of magnitude shorter in duration than the signal error thatresults from clipping of the laser and the subsequent ceasing of thelasing of the laser which causes the laser to lose its laser lightcoherency.

Since the signal receiver and demodulator typically integrate thereceived signal over a time period, which is typically orders ofmagnitude longer than the length of time of the large PAPR instantaneouspeak events, the receiver will typically encounter a signal error whichis orders of magnitude smaller when pre-clipping is utilized, relativeto the signal error encountered when the laser ceases lasing. Thisvastly improves the bit error rate (BER) performance achieved by thereceiver/demodulator, which is strongly affected by sporadic events ofhigh signal error (burst noise), and results in an improved performanceof the whole optical communication link.

When a pre-clipping scheme is implemented on the RF signal applied tothe laser, often the maximum OMI achievable by the laser can besubstantially higher than that possible without a pre-clipping scheme,thus increasing the laser effective RF modulated output power, whilemaintaining a similar total non-linear distortion and/or BER at thereceiver and demodulator receiving the laser output signal.Alternatively, the same OMI can be used, but with substantially reducednon-linear distortion and/or BER. Thus, applying pre-clipping in an RFcommunication laser transmitter may improve the operating dynamic rangeof the optical communication link by removing the memory effectassociated with laser clipping.

Any practical electronic system, such as an RF power amplifier (RFPA),has a limited dynamic range, and thus it may introduce noise anddistortion to the signal it processes when the signal is outside of thatdynamic range. While the largest possible dynamic range, or often in theRFPA case the largest possible output power, is desirable to reducenoise in the overall system, the dynamic range is often limited due toRFPA generated distortions, power consumption, cost, the availabletechnology, and other constraints.

The signal amplitude applied to the typical RFPA is adjusted such thatthe average (or RMS) power value is suitable to the capability of theRFPA to operate without generating distortions above an acceptablelevel. That adjustment typically targets a certain probability for thesignal PAPR peak to be higher than the RFPA maximum possible amplitude.That RFPA maximum possible amplitude is the amplitude of the RF signalat the RFPA output beyond which the RFPA produces a catastrophicdegradation in the amount of RF signal distortion.

In a prior art RFPA, when a very high PAPR instantaneous signal peakoccurs that is above the RFPA maximum possible amplitude, a signal erroris introduced to the signal at the output of the RFPA. Such a signalerror may manifests itself as a wide band noise that spreads over thein-band operating bandwidth of the communicating device, as such signalerrors are often restricted from also affecting the out-of-bandfrequency range by the use of analog blocking filters. The need toprevent these low probability of occurring peaks from creating noiseevents in-band and out-of-band is a motivation to set the RF signalamplitude such that the signal RMS level for that RF signal is at arelatively large back-off level relative to the RFPA maximum possibleamplitude for the RFPA.

However, given that the RFPA maximum possible amplitude is a trait ofthe RFPA, the higher the RMS signal back-off is from the RFPA maximumpossible amplitude, the lower the RF signal output power achievable bythe RFPA. Thus, there typically is an optimization implemented tocompromise between the large PAPR value exceeding the RFPA maximumpossible amplitude (which creates a short duration broadband noise eventas explained above) which have a low likelihood of occurring and theoverall maximum output power achievable by the RFPA.

It is desirable to increase the RFPA maximum possible amplitude toenable the handling of very high PAPR signals while keeping thebroadband noise generated by those signals' peaks very low relative tothe RF signal RMS value. However, there are limitations in availabletechnology to achieve this goal. Given the state of semiconductortechnology, there is a steep trade off in energy efficiency (powerconsumption) or distortion level to achieve even a small increase inRFPA possible RF signal output power.

Several traits of an RF power amplifier (RFPA) behavior are similar tothe DFB laser. An RFPA typically has a linear operating range thatgenerates little non-linear distortions and is typically DC biased suchthat when the RF signal is applied, the total signal generally staysinside the linear RFPA range. When the instantaneous total input (DCbias plus RF signal) goes outside the linear zone, there exists gracefullinearity degradation, in which the further the instantaneous totalinput signal departs from the linear zone, the more undesirablenon-linear distortions are generated by the RFPA.

Unlike the DFB laser, the RFPA linear range is typically only bound inthe lower input side of the graph. Setting a higher DC operating biascan increase the span of the RFPA linear operating range, but increasesthe power consumption of the RFPA. Similarly, increasing the powersupply voltage applied to the RFPA can increase the top range of the RFsignal output from the RFPA, but that too increases the powerconsumption of the RFPA. Thus, a tradeoff exists between the desire toreduce power consumption (RFPA DC current bias and power supplyvoltage), and achieve the maximum RF output power, while maintaining anoutput signal with a certain desirable maximum distortion level.

As with a DFB laser, when the instantaneous input signal is larger thana certain level, a RFPA may experience a clipping-like abruptcatastrophic deterioration in linearity, generating a very large amountof non-linear distortions. The input signal level that causes the largeamount of non-linear distortions is directly associated with the RFPAoutput RF signal maximum possible amplitude. Depending on theconstruction of the RFPA, there can also exist a memory effect in thereaction of the RFPA to an extreme instantaneous input signal. Thismemory effect may be caused by several different physical phenomena. Forexample, the instantaneous extreme input signal may result in tearingdown a depletion region required for an amplifier's operation. Thatdepletion region may take much longer to be rebuilt, after the inputreturns to normal condition, than the duration of the high PAPRinstantaneous input signal peak itself. During the time that thedepletion region is incomplete, the RFPA response (e.g., gain) severelydiffers from its normally expected behavior such that the actual outputsignal contains a large signal error relative to the expected outputsignal.

In this way, a very short instantaneous departure of the RF input signalfrom acceptable range causes a memory effect, stretching the length oftime that signal error and/or non-linear distortions appear at the RFPAoutput. A very short instantaneous departure of the RF input signal froman acceptable range may also severely increase signal error and/ornon-linear distortions' magnitude.

FIG. 7 is a plot of desired and actual RF signals at the output of anRFPA during a clipping induced memory effect in accordance with theprior art. As shown in FIG. 7, original large PAPR peak 710 exceeds cliplevel 720. After original large PAPR peak 710, the output signaldeviates from the desired signal for some time in response to originallarge PAPR peak 710 exceeding clip level 720.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are illustrated by way of example, and notby way of limitation, in the figures of the accompanying drawings and inwhich like reference numerals refer to similar elements and in which:

FIG. 1 illustrates such a PAPR plot for a typical CATV signal composedof 128×6 MHz channels of 256-QAM modulation in accordance with the priorart;

FIG. 2 illustrates a time domain linear plot of a typical CATV(t) signalcomposed of 128×6 MHz channels of 256-QAM modulation in accordance withthe prior art;

FIG. 3 is an absolute linear value plot of the same signal(ABS(CATV(t))) shown in FIG. 2 in accordance with the prior art;

FIG. 4 depicts the same signal shown by FIG. 2 in logarithmic scale(20*log 10(ABS(CATV(t)))) in accordance with the prior art;

FIG. 5 depicts, in the CATV(t) signal of FIG. 2, 100 samples immediatelynear the highest PAPR in accordance with the prior art;

FIG. 6 is a graph of laser bias current vs. optical output power inaccordance with the prior art;

FIG. 7 is a plot of desired and actual RF signals at the output of anRFPA during a clipping induced memory effect in accordance with theprior art;

FIG. 8 is a graph that compares the signal error created by clipping atan RFPA due to the difference between the desired and actual RF signalsat the output of an RFPA during a clipping induced memory event vs. thesignal error created by a pre-clipping memory-less mechanism inaccordance with an embodiment of the invention;

FIG. 9 is an illustration for an element that is capable of pre-clippingthe RF signal in accordance with an embodiment of the invention;

FIG. 10 is an illustration for an element that is capable ofpre-clipping the RF signal in the digital domain in accordance with anembodiment of the invention;

FIG. 11 is an illustration of a high PAPR peak prediction element thatmomentarily increases or supplements the normal bias circuit of a RFPAto prevent the RFPA from encountering a catastrophic memory event inaccordance with an embodiment of the invention; and

FIG. 12 is an illustration of an approach for preventing the RFPA outputvoltage from dropping below a pre-determined value in accordance with anembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Approaches for preventing RF signal distortion and signal errorproducing memory events in a Radio Frequency (RF) power amplifier (RFPA)are presented herein. In the following description, numerous specificdetails are set forth in order to provide a thorough understanding ofthe embodiments of the invention described herein. It will be apparent,however, that the embodiments of the invention described herein may bepracticed without these specific details. In other instances, well-knownstructures and devices are shown in block diagram form or discussed at ahigh level in order to avoid unnecessarily obscuring teachings ofembodiments of the invention.

Embodiments of the invention may be employed in a variety of contextsinvolving the transmission of RF signals that are susceptible to RFsignal distortion and signal error producing memory events. For example,embodiments may be employed within a communication device operablewithin a Hybrid Fiber Coax (HFC) system. Other embodiments of theinvention may be employed outside of a HFC system.

Embodiments of the invention discussed herein are directed towards theprevention of RF signal distortion and signal error producing memoryevents in a Radio Frequency (RF) power amplifier (RFPA). In anembodiment, an element, disposed prior to the Radio Frequency (RF) poweramplifier (RFPA) in a signal path of a RF signal input to the RFPA, mayenforce a maximum allowable amplitude in a high PAPR instantaneous highpeak of the RF signal. In another embodiment, an element may alsoincrease or supplement a bias of the Radio Frequency (RF) poweramplifier (RFPA) when a high PAPR instantaneous high peak is detected inthe RF signal prior to receipt by the RFPA. In a further embodiment, afirst element operable detects when an instantaneous output voltage ofthe Radio Frequency (RF) power amplifier (RFPA) is below a predeterminedvoltage, and in response, a second element supplies additional currentto prevent the output voltage of the RFPA from falling below apredetermined threshold voltage. These and other embodiments arediscussed in greater detail herein.

An embodiment of the invention may operate to eliminate theinstantaneous RF signal departure from an acceptable range by modifyingthe RF signal. This embodiment prevents the occurrence ofmemory-inducing clipping in the RFPA by ‘pre-clipping’ the RF signalbefore it is applied to the RFPA using a very fast (memory-less)element.

Pre-clipping, or modification of the RF signal to ensure a predeterminedamplitude is not exceeded, may be performed in various locations, e.g.,pre-clipping may be performed in the analog domain or the digitaldomain. FIG. 9 is an illustration for element 910 that is capable ofpre-clipping the RF signal in the analog domain in accordance with anembodiment of the invention. Element 910 pre-clips the RF signal in theanalog RF domain using a pair of Schottky diodes as shown in FIG. 9.

FIG. 10 is an illustration for element 1010 that is capable ofpre-clipping the RF signal in the digital domain in accordance with anembodiment of the invention. As shown in FIG. 10, digital pre-clippingelement 1010 performs pre-clipping of the RF signal in the digitaldomain prior to the digital-to-analog conversion of the RF by DAC 1020.Digital pre-clipping element 1010 should consider various signalconditioning elements that may be included in the communication deviceRF path between DAC 1020 and RFPA 1050, such as an anti-aliasing filter1040 commonly used after DAC 1020 and/or a preamplifier 1030.

Embodiments may perform a pre-clipping operation by initiallyestablishing a pre-clipping level (a threshold). The pre-clipping levelor threshold can be devised such that it directly corresponds to thelevel of the RF signal at the RFPA that causes the catastrophic memoryevent (e.g., considering the RF signal gain in the system and othereffects). Unlike in DFBs, since RFPAs are often constructed with dualamplifiers arranged in a differential push-pull structure, it is mostoften required to implement a symmetrical pre-clipping scheme in whichboth negative and positive RF peaks are limited to a certainpredetermined amplitude that will prevent both sides of the differentialRFPA from clipping. The positive edge pre-clipping prevents clipping inone of the differential amplifiers within the RFPA, and the negativeedge pre-clipping prevents clipping in the other amplifier within theRFPA.

Elements that perform pre-clipping should be fast reacting, such thatthose elements not only react fast enough to clip high peaks withoutletting any signal amplitudes above the predetermined level proceed tothe RFPA, but also restore the output signal to its intended levelimmediately after the RF signal goes below the predetermined level, thuscreating no memory effect.

Performing pre-clipping on the RF signal prior to the RF signal beingpropagated to the RFPA results in a signal error at the input to theRFPA whenever an instantaneous high PAPR peak is higher than thepre-clipping threshold. At the actual time that a pre-clipping elementis operational, the signal error that the pre-clipping element generatesmay be even slightly larger than the signal error that would have beengenerated by the RFPA if pre-clipping was not performed. This is sobecause an implementation margin may be needed to make sure that theRFPA clipping will not occur. However, the signal error resulting from apre-clipping operation is limited to a very short duration and is notextended by an RFPA memory effect. Accordingly, the overall integratederror created by the pre-clipping performed by an embodiment can beseveral orders of magnitude smaller than if embodiments did not performpre-clipping.

FIG. 8 is a graph that compares the signal error created by clipping atan RFPA due to the difference between the desired and actual RF signalsat the output of an RFPA during a clipping induced memory event vs. thesignal error created by a pre-clipping memory-less mechanism inaccordance with an embodiment of the invention. As shown in FIG. 8, thesignal error produced by the performance of the pre-clipping operation(the lower graph) is slightly larger than the signal error that wouldhave been generated by the RFPA if pre-clipping was not performed (theupper graph). However, the upper graph demonstrates that the signalerror generated by the RFPA if pre-clipping was not performed persistsfor a substantial longer duration than that shown in the lower graph inFIG. 8.

The bit error rate (BER) experienced at a receiver/demodulator whichreceives the RF signal delivered by the RFPA is affected by theintegrated signal error duration. Accordingly, a very short signal errorevent (resulting from clipping prevention obtainable by an embodiment)has a much smaller effect on the receiver than the effect of a stretchedsignal error due to the memory effect of a clipped amplifier (asexperienced by the prior art). Thus, clipping prevention of anembodiment results in improving the robustness of the transmission linkand can largely lower the probability that a very high PAPR peak eventin the RF signal will result in creating bit errors at thereceiver/demodulator.

Alternatively, the reduction in the amount of signal error in the RFPAoutput enabled by a clipping prevention scheme of an embodiment mayinstead be traded off with reducing the back off enacted relative to theRFPA maximum possible amplitude for purposes of increasing the RMSsignal level at the RFPA. Doing so results in more events where a largepeak PAPR signal passes the pre-clipping threshold (due to the higherlikelihood of such events coupled with the lower back off), and thusmore events of signal error are generated by the pre-clipping element,and with even higher excursion of the large peak PAPR over thepre-clipping level than before. However, due to the lack of a memoryeffect and the short duration of these signal errors, the totalresulting receiver BER can be maintained at sufficiently low levels,thus resulting in acceptable reception performance at the receiver.Thus, the clipping prevention performed by an embodiment can allow forincreasing the RF signal output power without consuming more power atthe RFPA (improving RFPA power efficiency), resulting in power savingand without worsening the whole system's BER.

Embodiment of the invention may also operate to prevent RF signaldistortion and signal error producing memory events in a Radio Frequency(RF) power amplifier (RFPA) by using an element to increase orsupplement a bias of a Radio Frequency (RF) power amplifier (RFPA) whena high PAPR instantaneous high peak is detected in the RF signal priorto receipt by the RFPA. In contrast to embodiments which modify the RFsignal to eliminate high PAPR instantaneous high peaks of the RF signal,certain embodiments prevent the RFPA from encountering a catastrophicmemory event by ensuring the RFPA does not exit its intended operationalrange (i.e., its dynamic range) and does not enter a non-operatingcondition. In this way, the memory effect associated with an undesirabledelay in reentering the desired operating position is avoided. Tofurtherance of this goal, embodiments may momentarily increase orsupplement the normal bias circuit of the RFPA. That momentary biasincrease or supplementation is designed to have negligible effect on theRFPA operation during normal operation and is only enacted whenever ahigh PAPR instantaneous high peak occurs.

FIG. 11 is an illustration of high PAPR peak prediction element 1102capable of momentarily increasing or supplementing the normal biascircuit 1122 of RFPA 1120 to prevent RFPA 1120 from encountering acatastrophic memory event in accordance with an embodiment of theinvention. FIG. 11 depicts a direct modification of the RFPA DC bias1122. RFPA DC bias 1122 is increased when a high PAPR instantaneous highpeaks is detected in the input RF signal to RFPA 1120. Since the biascircuit of RFPA 1120 normally has a slower response than RF signalresponse, a delay can be added to the RF signal to permit the biasincrease to come into effect by the time the detected high PAPR peakreaches RFPA 1120. The high PAPR peak can be detected by sampling the RFsignal using an RF coupler and comparing the RF signal to a predefinedclip level above which the bias will be increased. The bias can be thusgradually increased (using a low pass filter) from its normal level to aboosted level and then reduced back to the normal level. Element 1102will match the signal propagation delay in the two paths of the system(RF signal path 1124 and RFPA bias path 1122) such that the RFPA isbiased at its peak when the high PAPR peak signal reaches RFPA 1120.While the length of time of increased bias to RFPA 1120 is considerablylonger than the actual high PAPR instantaneous peak, a carefulimplementation of element 1102 can achieve negligible signal errorgenerated during the entire time the bias to RFPA 1120 is increased,including the time of the high PAPR instantaneous peak itself.

The bias increase into RFPA 1120 also increases the power consumption ofRFPA 1120. However, that power consumption increase is of shortduration, and since high PAPR events have a low probability of occurringand are typically spaced enough apart in time, the average powerconsumption increase can be negligible. The momentary power increase canbe supplied by a DC-line capacitor, or other like sources, so that thepower supply which provides power to the RFPA experiences negligibleadditional load during the time that the bias is increased.

FIG. 11 depicts bias modification for a single bias line only appliedfor a single peak polarity of high PAPR peak signals in accordance withan embodiment of the invention. However, since RFPA are oftenconstructed with dual amplifiers arranged in a differential push-pullstructure, an implementation can detect both positive and negative highPAPR peak signals to modify the bias of both differential amplifiers inan RFPA, or even detect positive high PAPR peak signals to modify thebias of one of the two differential amplifiers in an RFPA, and detectnegative high PAPR peak signals to modify the bias of the other.

Embodiments of the invention may also operate to prevent RF signaldistortion and signal error producing memory events in a Radio Frequency(RF) power amplifier (RFPA) by preventing the RFPA output voltage fromdropping below a pre-determined value. Whether the RFPA is based onheterojunction bipolar transistor (HBT), metal-semiconductorfield-effect transistor (MESFET), pseudomorphic high electron mobilitytransistor (pHEMT) or other transistor technologies with transistorsarranged in cascade or other formations, the RFPA will typically exitits proper operation zone if, due to a high instantaneous signal peak inthe RF signal, the instantaneous output voltage reaches below a certainvoltage. This can happen due to transistor saturation, reversal of agate-source voltage, or other reasons.

FIG. 12 is an illustration of an approach for preventing the RFPA outputvoltage from dropping below a pre-determined value in accordance with anembodiment of the invention. FIG. 12 depicts an implementation thatprevents a class-A differential push pull amplifier from reaching toolow of an output voltage. A pair of fast acting diodes (e.g., Schottkydiodes) 1202 is connected to the RFPA differential outputs. When aninstantaneous high peak signal causes any of the RFPA outputs' voltageto drop below a predetermined voltage, VDS_(min), maintained in thisembodiment on capacitor 1204, the corresponding diode switches on andcapacitor 1204 can supply the additional current required to keep theRFPA output voltage from dropping further, thus preventing the RFPA fromexiting its operating zone.

Capacitor 1204 only needs to provide the additional current to the RFPAtransistor for a very short duration (i.e., the duration of theinstantaneous peak). Right after the peak duration, the required RFPAtransistor current goes down, enabling the RFPA output voltage to riseabove VDS_(min), thus the diode turns back off. Capacitor 1204 may nowslowly replenish its spent charge from the RFPA DC power line through aresistor network or other recharging means.

The length of time that additional current is provided to the RFPA isonly so long as the high instantaneous signal peak duration, and thatadditional current is supplied by capacitor 1204. The average powerconsumption is negligibly affected, due to the slow capacitor chargingprocess, and increases the demand from the DC power source in aninsubstantial manner.

Although the RFPA itself does not enter a clipping related memory event,by not allowing the RFPA output voltage to go below VDS_(min), an actualsignal clipping is caused at the output of the RFPA amplifier. Theduration of this signal clipping is similar to the duration of the largePAPR peak event, and it does create a very short duration of signalerror at the RFPA output, similar in amplitude and duration to thatsignal error created by the pre-clipping scheme. Accordingly,embodiments of the invention that employ output voltage restrictionpresent all the same benefits of the pre-clipping embodiments discussedabove. For example, RFPA clipping prevention can result in improving therobustness of the transmission link by reducing BER at thereceiver/demodulator. Alternatively, BER reduction can be traded offwith increasing the RMS signal level at the RFPA, increasing the RFsignal output power, without consuming more power at the RFPA, andwithout worsening the whole system's BER.

In the foregoing specification, embodiments of the invention have beendescribed with reference to numerous specific details that may vary fromimplementation to implementation. Thus, the sole and exclusive indicatorof what is the invention, and is intended by the applicants to be theinvention, is the set of claims that issue from this application, in thespecific form in which such claims issue, including any subsequentcorrection. Any definitions expressly set forth herein for termscontained in such claims shall govern the meaning of such terms as usedin the claims. Hence, no limitation, element, property, feature,advantage or attribute that is not expressly recited in a claim shouldlimit the scope of such claim in any way. The specification and drawingsare, accordingly, to be regarded in an illustrative rather than arestrictive sense.

What is claimed is:
 1. An apparatus for preventing RF signal distortionand signal error producing memory events in a Radio Frequency (RF) poweramplifier (RFPA), comprising: the Radio Frequency (RF) power amplifier(RFPA), wherein the RFPA resides in a communication device operablewithin a wireless system, a cellular system, or a Wi-Fi system; and anelement, disposed prior to the RFPA in a signal path of a RF signalinput to the RFPA, operable to enforce a maximum allowable amplitude ina high peak to average power ratio (PAPR) instantaneous high peak of theRF signal.
 2. The apparatus of claim 1, wherein the element does notexperience the signal error producing memory events, and wherein saidsignal error producing memory events cause distortions in the RF signalover time in response to the RF signal having a high PAPR instantaneoushigh peak beyond a threshold amplitude.
 3. The apparatus of claim 1,wherein the maximum allowable amplitude corresponds to a determinedamplitude above which a signal error producing memory event is inducedin said RFPA.
 4. The apparatus of claim 1, wherein the maximum allowableamplitude is configured to be an amplitude that is less than adetermined amplitude above which a signal error producing memory eventis induced in said RFPA.
 5. The apparatus of claim 1, wherein themaximum allowable amplitude is enforced against both positive amplitudepeaks and negative amplitude peaks of the RF signal.
 6. The apparatus ofclaim 1, wherein the maximum allowable amplitude is enforced in ananalog domain.
 7. The apparatus of claim 6, wherein the enforcement ofthe maximum allowable amplitude in the analog domain is performed usingone or more Schottky diodes.
 8. The apparatus of claim 6, wherein theenforcement of the maximum allowable amplitude in the analog domain isperformed using a pair of Schottky diodes.
 9. The apparatus of claim 1,wherein the maximum allowable amplitude is enforced in a digital domainprior to being received at a digital-to-analog converter (DAC).
 10. Anapparatus for preventing RF signal distortion and signal error producingmemory events in a Radio Frequency (RF) power amplifier (RFPA),comprising: the Radio Frequency (RF) power amplifier (RFPA), wherein theRFPA resides in a communication device operable within a wirelesssystem, a cellular system, or a Wi-Fi system; and an element operable toincrease or supplement a bias of the RFPA when a high PAPR instantaneoushigh peak is detected in the RF signal prior to receipt by the RFPA. 11.The apparatus of claim 10, wherein the element is a first element, andwherein the apparatus further comprises: a second element, disposedprior to the RFPA in a signal path of a RF signal input to the RFPA,operable to detect the high PAPR instantaneous high peak in the RFsignal by comparing a measured amplitude of the RF signal to apredetermined amplitude.
 12. The apparatus of claim 10, wherein theelement increases or supplements the bias of the RFPA gradually using alow pass filter from a normal level to an elevated level while the highPAPR instantaneous high peak is being processed by the RFPA, and whereinthe element decreases the bias of the RFPA gradually from the elevatedlevel to the normal level when the high PAPR instantaneous high peak isno longer being processed by the RFPA.
 13. The apparatus of claim 10,wherein said RFPA comprises a first amplifier and a second amplifierdifferent than the first amplifier, and wherein the bias of each of thefirst and second amplifier is modified independently of the other. 14.An apparatus for preventing RF signal distortion and signal errorproducing memory events in a Radio Frequency (RF) power amplifier(RFPA), comprising: the Radio Frequency (RF) power amplifier (RFPA),wherein the RFPA resides in a communication device operable within awireless system, a cellular system, or a Wi-Fi system; a first elementoperable to detect when an instantaneous output voltage of the RFPA isbelow a predetermined voltage; and a second element operable to supplyadditional current to prevent the output voltage of the RFPA fromfalling below a predetermined threshold voltage.
 15. An apparatus ofclaim 14, wherein the second element comprises a capacitor whichrecharges from a DC power line of the RFPA or another DC power line. 16.An apparatus of claim 14, wherein the first element comprises one ormore Schottky diodes.
 17. An apparatus of claim 14, wherein the secondelement supplies said additional current only over a durationcommensurate to a length of time in which a RF signal that is input tothe RFPA has a high PAPR instantaneous high peak beyond a thresholdamplitude.
 18. A method for preventing RF signal distortion and signalerror producing memory events in a Radio Frequency (RF) power amplifier(RFPA), comprising: an element, disposed prior to the Radio Frequency(RF) power amplifier (RFPA) in a signal path of a RF signal input to theRFPA, enforcing a maximum allowable amplitude in a high peak to averagepower ratio (PAPR) instantaneous high peak of the RF signal, wherein theRFPA resides in a communication device operable within a wirelesssystem, a cellular system, or a Wi-Fi system.
 19. A method forpreventing RF signal distortion and signal error producing memory eventsin a Radio Frequency (RF) power amplifier (RFPA), comprising: an elementincreasing or supplementing a bias of the Radio Frequency (RF) poweramplifier (RFPA) when a high PAPR instantaneous high peak is detected inthe RF signal prior to receipt by the RFPA, wherein the RFPA resides ina communication device operable within a wireless system, a cellularsystem, or a Wi-Fi system.
 20. A method for preventing RF signaldistortion and signal error producing memory events in a Radio Frequency(RF) power amplifier (RFPA), comprising: a first element operable todetect when an instantaneous output voltage of the Radio Frequency (RF)power amplifier (RFPA) is below a predetermined voltage; and in responseto the first element detecting that instantaneous output voltage of theRFPA is below a predetermined voltage, a second element operable tosupply additional current to prevent the output voltage of the RFPA fromfalling below a predetermined threshold voltage, wherein the RFPAresides in a communication device operable within a wireless system, acellular system, or a Wi-Fi system.